1. Field
This invention relates to the field of receiver circuits, more particularly to a method and apparatus for receiving signals in which a portion of a transmitted signal forms a guard interval.
2. Description of Related Art
The European DVB-T (Digital Video Broadcasting-Terrestrial) standard ETS 300 744 for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore grouped into blocks and frames.
It is necessary to transmit the DTT signals over transmission paths, which are of uncertain quality. In particular, the area close to the transmission path may include objects such as tall buildings, which cause reflections. That is, a signal may be received at a receiver twice, once on a direct path from the transmitter, and then, after a short delay, as an indirect reflected path. As is well known, this can cause inter-symbol interference (ISI) in the receiver. To minimize this problem, DVB-T COFDM signals include a cyclic prefix guard interval (referred to hereinafter as “guard interval” or “guard period”) to each active symbol. Specifically, the end portion of the active symbol is repeated before the current active symbol. These cyclic signals must be removed correctly before demodulation or the demodulation performance can be seriously degraded.
The DVB-T standard defines two possible “modes” of operation, each of which has a specific active window size, which in turn defines the size of the Fast Fourier Transform (FFT) in the receiver, namely 2 k mode and 8 k mode. For each of these modes the standard defines four possible guard lengths, which are fractions of the respective FFT length. The combination of FFT size and guard interval length can be selected based upon the anticipated multipath delay spread (which can be a function of transmission path topology and/or network topology in the case of a single frequency network (SFN)).
Before the cyclic signals can be removed it is necessary to know the FFT size and the duration of the guard interval. However, the duration of the guard interval, at least, is not a constant value. The presence of the guard interval reduces the efficiency with which the transmission channel can be used, and so it is advantageous to vary the size of the guard interval to be the minimum allowed by the conditions.
The receiver therefore includes a mechanism for determining the size of the guard interval. The chosen mechanism should be sufficiently robust to perform adequately under noisy signal conditions and with signals that suffer from multipath and co-channel interference.
Therefore, a need exists for a method and apparatus for a multicarrier receiver circuit with guard interval size detection that can be easily implemented and overcomes the disadvantages of other methods and apparatuses such as the above-described known system. The present disclosure provides such a multicarrier receiver circuit method and apparatus.